1. Field of the Invention
The present invention relates to gain calibration methods and devices and, more particularly, is directed towards a method and apparatus for easily and quickly calibrating the gain of one or a plurality of photomultiplier tube-preamplifier combinations.
2. Description of the Prior Art
Certain video systems are known which employ a light source, such as a laser, to scan and therefore illuminate an object or scene desired to be displayed. The light which is scattered from the scene is detected by a plurality of light detectors, such as photomultiplier tubes, which are normally arranged in triads, one for each primary color. Since the light is scattered, the outputs from the photomultiplier tubes must be summed, from which the image may be reconstructed in the three primary colors.
Each photomultiplier tube has a relatively low level output which must be raised to system video levels (for example, one volt peak-to-peak). This must be accomplished with very low noise and a wide, uniform bandwidth of, for example, 0-40 megahertz. To accomplish these functions, associated with each photomultiplier tube is a preamplifier connected to receive the output signal of the tube and amplify it in accordance with these desired characteristics.
Since photomultiplier tubes exhibit relatively poor gain stability, they must be frequently calibrated in order to insure a uniform output from day-to-day throughout the system.
Known methods utilized to control gain in a preamplifier include the use of a motor-driven potentiometer or an automatic gain control circuit in an open loop configuration. Such techniques require manual calibration of gain and are relatively tedious and time consuming. As a result, readiness maintenance of the preamplifier is infrequently performed, and gain accuracy leaves much to be desired, especially when the preamplifier is connected to a signal source such as a photomultiplier tube. Another technique utilized to adjust the gain of a photomultiplier tube is to vary the high voltage to its dynode divider string. This has been accomplished by using a servo control and a suitable reference. Such a technique is acceptable, however, only when a relatively small number of photomultiplier tube-preamplifiers are involved. In a laser scanner system, as described above, a large number of preamplifiers are involved, and such techniques become extremely cumbersome.
The following U.S. patents relating to this general area of technology are known: U.S. Pat. Nos. 3,900,731; 4,036,763, and 4,127,813.